Development within wireless technology has been, and still is, on rampage. The use of wireless communications networks, sometimes also referred to as cellular communications networks, cellular radio system or cellular networks, continues to grow rapidly. New wireless technologies and standards are constantly emerging. Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM) and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access. The 3rd Generation Partnership Project Long Term Evolution (3GPP LTE), which may be referred to as Evolved UTRAN (E-UTRAN), has undertaken to evolve further the UTRAN and GSM based radio access network technologies, and 3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic.
A user equipment (UE) which for instance may be represented by a mobile terminal, wireless terminal or mobile station, may be enabled to communicate wirelessly in any of such wireless communications networks. A wireless communications network may cover a geographical area which is divided into cell areas, wherein each cell area is served by a base station. A cell is the geographical area where radio coverage is provided by the base station at a base station site. Each base station may serve one or several cells, and furthermore, each base station may support one or several communication technologies and be directly connected to one or more core networks. Depending on the technology and terminology used, a base station may be referred to as e.g. a Radio Base Station (RBS), Base Transceiver Station (BTS), B node, NodeB, Evolved Node B (eNodeB), or eNB, and the term “base station” is used in this description to denote any of these. The base stations may communicate over the air interface operating on radio frequencies with the UEs within range of the base stations.
A UE may be subjected to handover from one cell to another. One reason may for instance be that more efficient utilization of capacity is sought after, and another that the UE is moving away from an area covered by one cell—the source cell—and is entering an area covered by another cell—e.g. a target cell—which therefore provides better radio conditions for the UE.
Commonly, a UE in idle mode measures the signal level of cells in the current frequency and also signal levels of cells in other frequencies and other Radio Access Technologies (RATs). Different frequencies and RATs may be prioritized differently. This prioritization is provided by a base station, such as e.g. an eNB, to the UE as a part of System Information (SIB) or by an RRCConnectionRelease message. The UE in idle mode is supposed to follow this prioritization when performing measurements for cell selection or reselection.
Measurements on lower priority frequencies/RAT are commonly only performed when the serving cell's signal level is less than a defined threshold. Measurements on higher prioritized frequencies/RATs are commonly always performed.
If several cells are found on frequencies with equal priority, the strongest one is selected, considering the configured offsets and hysteresis. The cell selection criterion, S criteria, and cell ranking criterion, R criteria, are specified in 3GPP TS 36.304, “User Equipment (UE) procedures in idle mode”, Rel-11 (V11.2.0).
In a prior art example configuration illustrated in FIG. 1, a cell 1, which is located on a first frequency f1, and a cell 2, which is located on a second frequency f2, are assumed to be macro cells with generally moderate/high load. Cell 3 is a pico cell, located on the same second frequency f2 as cell 2, adding extra capacity in its coverage area.
The coverage area of a pico cell like cell 3 is often small. Fully utilizing the capacity of a pico cell might not be possible, because the number of UEs within coverage is too small. It is, however, desired that all UEs located within the coverage area of cell 3 shall camp on this cell, in order to utilize the capacity, and in order to offload the macro cells 1 and 2, as far as possible.
Connected mode load balancing is commonly applied between the macro cells 1 and 2. It distributes the UEs evenly between those two cells, ensuring consistent end user performance in these two cells. The idle mode cell reselection in these two cells is typically configured to preserve the UE distribution the load balancing achieves in connected mode. It reduces the need for recurring load balancing actions when the UEs reconnect after periods in idle mode.
In connected mode, UEs connected to cell 2 are commonly relocated to cell 3 when entering the cell 3 coverage. This is achieved with intra-frequency mobility in connected mode, with support in the existing technology.
UEs connected to cell 1, on the other hand, tend to stay connected to cell 1, although they may enter the coverage of cell 3. One way to move UEs to cell 3 is by means of the connected mode load balancing. However, since cell 3 has a relatively small coverage area, it requires a lot of inter-frequency measurements in cell 1 in order to identify UEs within coverage of cell 3 hence suitable for relocation.